The murine microphthalmia gene (mi) is one of the last multi-allelic, classic coat-colour genes to be cloned in the mouse and, similar to many of these genes, encodes an exciting molecule that is involved in multiple developmental processes. The existence of the numerous alleles has allowed the molecular dissection of the function of the MI bHLH-Zip transcription factor in vivo and offers a unique opportunity to understand the function of a multimeric transcription factor throughout development and in many tissues. It is also the gene mutated in some patients with the human deafness syndrome, Waardenburg's syndrome type II, and hence helps to understand this syndrome.